1. Field of the Invention
The present invention relates to a photothermographic material preferably used in the field of films for medical diagnosis, in the field of films for graphic arts, or the like.
2. Description of the Related Art
In recent years, in the field of films for medical diagnosis and in the field of films for graphic arts, there has been a strong desire for decreasing the amount of processing liquid waste from the viewpoints of protecting the environment and economy of space. Technology is therefore required for light sensitive photothermographic materials which can be exposed effectively by laser image setters or laser imagers and thermally developed to obtain clear black-toned images of high resolution and sharpness, for use in medical diagnostic applications and for use in photographic technical applications. The light sensitive photothermographic materials do not require liquid processing chemicals and can therefore therefore be supplied to customers as a simpler and environmentally friendly thermal processing system.
While similar requirements also exist in the field of general image forming materials, images for medical imaging in particular require high image quality excellent in sharpness and granularity because fine depiction is required, and further require blue-black image tone from the viewpoint of easy diagnosis. Various kinds of hard copy systems utilizing dyes or pigments, such as ink jet printers and electrophotographic systems, have been marketed as general image forming systems, but they are not satisfactory as output systems for medical images.
Thermal image forming systems utilizing organic silver salts are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, as well as in “Thermally Processed Silver Systems” by D. H. Klosterboer, appearing in “Imaging Processes and Materials”, Neblette, 8th edition, edited by J. Sturge, V. Warlworth, and A. Shepp, Chapter 9, pages 279 to 291, 1989. (All patents, patent publications and non-patent literature cited in this Specification are hereby expressly incorporated by reference herein in their entirety.) In particular, photothermographic materials generally have an image forming layer including a catalytically active amount of a photocatalyst (for example, silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and if necessary, a toner for controlling the color tone of developed silver images, dispersed in a binder. Photothermographic materials form black silver images by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed on the exposed region.
This type of photothermographic material is well known, and the image forming layer in many of these recording materials is prepared by a process using organic solvents such as toluene, methyl ethyl ketone, or methanol as a solvent. However, use of an organic solvent as a solvent is not advantageous, not only in view of undesired effects on the human body during manufacturing steps, but also in view of the cost due to recovery of solvents, and the like.
In view of the above, a method for preparing an image forming layer using a coating solution with an aqueous medium has been disclosed. For example, a technique for utilizing gelatin as a binder has been disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 49-52626, and 53-116144. Further JP-A No. 50-151138 discloses a technique for utilizing poly(vinyl alcohol) as a binder.
However, the above techniques often lead to an increase in fogging and therefore hardly attain a desired sensitivity. Moreover, the resultant image tone is not preferred.
On the other hand, JP-A Nos. 10-10670 and 10-62899 disclose a method for preparing an image forming layer using a polymer as a binder and an aqueous medium.
JP-A No. 2002-303953 discloses a technique for utilizing a polymer latex having a specific physical character as a binder to improve manufacturing-related brittleness and image storability under dark storage conditions (fogging during storage) of photosensitive materials. JP-A No. 11-84573 discloses a technique for utilizing a specific polymer latex as a binder for the image forming layer and a protective layer to attain low fog and high Dmax.
However, the techniques described above do not sufficiently improve sensitivity and fog of photothermographic materials. Therefore, further improvements are demanded. Furthermore, with regard to varying factors in a thermal developing process, especially variations in temperature for thermal development, an improvement in processing stability of photothermographic materials is further required. In particular, diagnostic performance of the photothermographic materials utilized for use in medical diagnosis is also largely influenced by such factors, and therefore high image quality is always demanded.